Method of maintaining aluminum in contact with molten alkali-metal hydroxides and carbonates



May 26, 1970 R. L. NOVACK 3,514,334

METHOD OF MAINTAINING ALUMINUM IN CONTA WI MOLTEN ALKALI-METAL HYDROXICAR A" Original Filed e 1966 ML UM/A/UM/ MOLTE/V AL/(AU-MEML HYDROX/DE &

ROBERT L. NOVACK, INVENTOR ATTORNEYS United States Patent 3,514,334METHOD OF MAINTAINING ALUMINUM IN CONTACT WITH MOLTEN ALKALI-METALHYDROXIDES AND CARBONATES Robert L. Novack, Arlington, Mass., assignorto Prototech Incorporated, Cambridge, Mass., a corporation ofMassachusetts Continuation of application Ser. No. 376,917, June 22,1964. This application Dec. 19, 1967, Ser. No. 691,912 Int. Cl. H01m27/00 US. Cl. 136-86 4 Claims ABSTRACT OF THE DISCLOSURE This disclosuredeals with the discovery that aluminum can be maintained in continuouscontact with molten alkali-metal hydroxide and carbonate media withoutcorrosive attack, by appropriate initial preparation of and maintenanceof the molten media in a substantially anhydrous state, while insuringsuch contact only in such state.

This application is a continuation of application 376,- 917 filed June22, 1964, now abandoned.

The present invention relates to methods of and apparatus for containingalkali-metal hydroxides and carbonates, being more specifically directedto a discovery that enables the utilization of aluminum for suchpurposes without the corrosive attack thereof.

It has long been accepted that aluminum is readily attacked in aqueousalkali-metal hydroxide solutions, forming a precipitate of aluminumhydroxide or forming aluminates. Similarly, the ready dissolution ofaluminum in hot concentrated solutions of alkali-metal carbonates, suchas sodium carbonate, has long been known. As a result, the art hasconsidered aluminum a most unsatisfactory metal for use in systems inwhich it must come into contact with alkali-metal hydroxides orcarbonates. In the alkali electrolyte fuel cell field, for example, ithas even been reported that aluminum cannot stand up to suchelectrolyte, particularly at elevated temperatures.

In accordance with the present invention, however, a discovery has beenmade that, under certain very critical conditions, aluminum may not onlywithstand attack from alkali-metal hydroxides and carbonates, but it maybe beneficially used even as a container for such hydroxides andcarbonates and even at elevated temperatures.

An object of the present invention, therefore, is to provide a new andimproved aluminum container and/ or contacting element for alkali-metalhydroxides and carbonates and a new and improved method of renderingaluminum resistant to corrosive attack by such hydroxides and carbonatesat elevated temperatures.

Other and further objects, including applications to novel fuel cellsand the like, will be explained hereinafter and will be moreparticularly pointed out in connection with the appended claims.

The invention will now be described in connection With the accompanyingdrawing, the single figure of which illustrates a preferred use ofaluminum in a molten alkali-metal hydroxide or carbonate fuel cell orthe like.

It has been discovered that if molten alkali-metal hydroxides andcarbonates are maintained at temperatures between about 300 C. and 500C., more or less, and the water content thereof is allowed to decreaseuntil it approaches substantial equilibrium with the moisture in the airor other medium surrounding the melt, aluminum, preferably substantiallypure (although aluminum with some impurities and alloys thereof havebeen found to exhibit some of the resistant characteristics herein3,514,334 Patented May 26, 1970 ice described), can be placed in directand continuous contact with the molten alkali-metal hydroxides andcarbonates without suffering any appreciable corrosion.

While it is not desired to predicate the invention upon a theory orhypothesis of operation, it being sufiicient to describe the inventionas it has been found to work in practice, it appears that the clue torendering aluminum resistive to alkali-metal hydroxides and carbonatesresides in the temperature elevation to the point of driving 013? thewater therein sufficiently to attain the substantial equilibriumabove-discussed, and the maintenance of the molten alkali-metalhydroxide and/or carbonate in the molten state. Physically, such moltenhydroxide and carbonate assumes a substantially clear form when thisregion of substantial equilibrium is reached. It is this substantiallyanhydrous molten alkali hydroxide or carbonate that apparently no longerattacks the base metal of the aluminum with whatever oxide coating maybe formed thereupon.

Thus, in the embodiment of the drawing, an aluminum cup is shown at :1,used for a purpose heretofore considered impossible; namely, as acontainer for a molten alkali-metal hydroxide or carbonate medium 5comprising the electrolytic medium of a fuel cell. In this embodiment, atubular Ag-Pd anode 3 is shown disposed in contact with the electrolyticmedium 5 for passing, for example, hydrogen fuel into the cell; and anickel or other cathode electrode 7 is shown disposed in theelectrolytic medium 5, in this case operable with the oxidant providedby the apparent conversion of molten hydroxide, for example, intoperoxide or superoxide in the region of the cathode 7. It is, of course,to be understood that other types of cathodes, including porous cathodesthrough which oxidant may be introduced from an external source, may beemployed, as may other anodes. In the case of the utilization ofnon-porous nickel, iron or similar cathode electrodes 7, however,operating with the peroxide or superoxide oxidant, it is in some casesdesirable to introduce agitation in the region of the cathode, as byblowing air therein. This is illustrated as effected with an inexpensivealuminum tube 9, which, by operation of the cell 5 in thebefore-described temperature region to a degree such that the waterwithin the medium 5 is decreased until it approaches equilibrium wherethe water content in the air at the top of the medium 5 (i.e.substantially anhydrous), is rendered resistant to attack by the medium5.

The support 7 for the cathode electrode may also, if desired, be ofaluminum, as may the tubular support 3 for the anode 3. In connectionwith the latter, this is most unusual since aluminum is one of the fewmetals which is not active as a cathode in peroxide-containing melts andcan thus be used at the anode.

As an example of actual experimental results, an aluminum cup 1, 2 /2inches in diameter by about 4 inches deep, has been successfullyoperated for several months as the container for a potassium hydroxidemolten electrolytic medium 5 maintained at a temperature in thebefore-mentioned temperature range, and intermittently operated as afuel cell in accordance with the construction illustrated in thedrawing; all with no noticeable attack upon the aluminum with its oxidecoating. Similar results have also been obtained by immersing aluminumin molten lithium, potassium and sodium carbonates maintained at theabove-mentioned temperatures to provide the said substantial equilibriumand anhydrous condition. Other alkali-metal molten hydroxides, includingsodium, have also been so used with aluminum, all with successfulresistance of the aluminum to the molten baths.

Clearly, the invention is not restricted to application in the importantfuel cell field, but it is widely applicable wherever it is desired tocontact aluminum with such molten alkali-metal hydroxides andcarbonates.

Further modifications will occur to those skilled in the art, and allsuch are considered to fall within the spirit and scope of the inventionas defined in the appended claims.

What is claimed is:

1. A method of maintaining an aluminum member in contact with anelectrolytic medium consisting essentially of at least one of the groupconstituted by alkali-metal hydroxides and carbonates, said medium beingnormally corrosive to said member, and of preventing corrosive attackupon said member by said medium during the utilization of said member,which comprises elevating the temperature of said medium to atemperature sufficient to render the same molten, maintaining saidmedium at such elevated temperature for a time sufiicient to cause themolten medium to become anhydrous, and continuously maintaining theanhydrous condition of said medium during the utilization of said memberand while said member is maintained in contact with said medium.

2. A method as claimed in claim 1 and in which the temperature of saidmedium is raised to the range substantially 300 degrees to 500 degreesC.

3. A method of operating an electrochemical cell including the followingelements: a cell container, a cell electrode, a support for a cellelectrode, and an element for introducing gas into the cell, at leastone of said elements being an aluminum member, and of preventingcorrosive attack upon said member by a normally corrosive electrolyticmedium which contacts said member, said medium consisting essentially ofat least one of the group constituted by alkali-metal hydroxides andcarbonates, which comprises elevating the temperature of said medium toa temperature sufiicient to render the same molten, maintaining saidmedium at such elevated temperature for a time sufiicient to cause themolten medium to become anhydrous, and continuously maintaining theanhydrous condition of said medium during the utilization of said memberand while said member is in contact with said medium.

4. A method as claimed in claim 3 and in which a fuel and an oxidant areintroduced into said medium at fuel and oxidant electrodes therein,respectively, an output current is drawn in a circuit connected to saidfuel and oxidant electrodes, and said corrosive attack is preventedduring the drawing of said current.

References Cited UNITED STATES PATENTS 2,969,315 1/1961 Bacon 136-863,138,490 6/1964 Tragert 13686 3,202,547 8/1965 Rightmire et a1. 136-863,251,718 5/1966 Hilton 13686 2,244,526 6/ 1941 Mackay 1486.11 3,146,1318/1964 Linden et a1. 13686 3,294,586 12/1966 Le Duc 136-86 FOREIGNPATENTS 100,100 3/ 1933 Japan.

ALLEN B. CURTIS, Primary Examiner H. A. FEELEY, Assistant Examiner US.Cl. X.R. l36-l6l; 204-39

